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than as sequential steps. QTL/marker associations, however,
need to be tattered through more intensive research efforts. It
would be essential to find out whether extremely tight linkages
between marker loci and QTL may lead to highly conserved
allele associations. If so, observation of change in marker allele
frequency in long-term selection experiments or determination
of markers that explain significant portions of the combining
ability variance in diallel or factorial crosses might reveal uni-
versally applicable markers. Another approach would be to per-
form QTL analyses in genetically broad-based random mating
populations by means of extremely saturated integrated genetic
marker maps. Direct QTL-allele-specific markers (such as
STS markers derived from cloned QTL alleles) are needed for
maximal efficiency of MAS. Even though success in terms of
cloning of QTL alleles is very limited, map-based cloning and
candidate gene approaches would increasingly facilitate isola-
tion and characterisation of agronomically significant QTLs,
owing to the fast advancement being made in genome sequenc-
ing of many plants. Cloning of genetic determinants of QTLs
is anticipated to bridge the missing link in our understanding
of the association among genotype and phenotype (Geiger and
Welz 1999).
MAS for drought stress tolerance in maize: CIMMYT inves-
tigators have made substantial efforts throughout the past three
decades for augmentation of pre- and post-flowering drought
tolerance in maize and up to certain limits, energetic signifi-
cant progress has been achieved for cultivating drought tol-
erance in CIMMYT maize germplasm through conventional
breeding, but still, the approach is slow and long. To accel-
erate the breeding procedure, molecular markers and QTL
information based on precisely managed replicated tests have
been used to show the potential to improve the issues related
to inconsistent and unpredictable onset of moisture stress or
the confounding impact of different stresses such as heat. For
this, first of all, a complex trait of drought tolerance was coun-
teracted into simpler components, such as an anthesis-silking
interval, that are closely associated with drought tolerance.
After that, CIMMYT conducted a series of experiments on
QTL analysis and MAS for transfer of drought tolerance to
tropical maize, and obtained encouraging results. An associ-
ate integrated strategy of QTL-mapping, MAS and functional
genomics are currently being employed to additionally provide
genomic information and tools to commendably complement
conventional selection for rising drought stress tolerance in
maize (Bänziger 2000).
